In the so-called H bridge, four semiconductor switches form the four legs and the load, which is typically a motor winding, forms the crossbar. When direct current (D) is applied to such a switch the four elements can be operated so as to provide a current through the load in either direction thus controlling its rotation direction. In driving a stepper motor a succession of current pulses can be supplied for incrementing the motor as desired.
FIG. 1 illustrates a typical prior art monolithic integrated circuit (IC) H bridge. DC is applied +to terminal 10 and- to ground terminal 11. Four NPN transistors 12-15 form the 14 four legs of the H bridge and motor 16 forms the crossbar. Since the motor winding is inductive four clamp diodes 17-20 are included to absorb any inductive kickback transients. In the H bridge configuration transistors 12 and 14 are referred to as the top-side drivers while transistors 13 and 15 are referred to as the bottom-side drivers. Logic elements 21-24 control the conduction of switch elements 12-15 in response to control signals applied at terminals 25-27. In order to monitor the H bridge current it has become common to include a series resistor 28 which passes the total bridge current. Typically, this is a separate low value precision resistor that is exterior to any IC construction. It is clear that any voltage drop across resistor 28 is parasitic in that it detracts from the voltage applied to motor 16. The voltage that appears across resistor 28 is integrated by resistor 29 and capacitor 30. Amplifier 31 senses the voltage across capacitor 30 and provides a bridge current readout at current sense terminal 32.
While FIG. 1 shows the use of NPN power transistors 12-15 such elements are not ideal. They require separate external clamp diodes and are slow to turn off. It has been found that if transistors 12-15 are replaced with diffused metal oxide semiconductor transistors (DMOST's) several clear advantages occur. First, a DMOST incorporates its own clamp diode and no separate diode is needed. The DMOST also has a very low on resistance and can switch rapidly. Finally, the DMOST can employ a built in current sense as shown in FIG. 2. Here DMOST 34 is shown schematically to include its built-in clamp diode 35. The power source 36 is connected to its backgate and is shown grounded in a bottom-side switching application. A load element 37 returns the drain to the positive supply terminal 10. I.sub.L flows in the load and in the main DMOST source 36 in response to a control bias applied to gate input 38. An additional sense source 39 is included in the DMOST structure. Typically, a DMOST is composed of a large number of individual low current segments which are connected in parallel by IC metallization. For example, several thousand low current elements can be joined together to create a high current DMOST suitable for use in a H bridge. A single such element, separate from the rest, can then provide a sense current that is a small controlled fraction of the current being passed in the large area device. An op-amp 40 has its inputs coupled to DMOST sources 36 and 39. A resistor 41 is coupled from the output of op-amp 40 to its inverting input to provide negative feedback to the inverting input which is connected to the sense source 39. Thus, op-amp 41 will drive output terminal 42 until the potentials on sources 36 and 39 are equal. Under this condition I.sub.S will flow in resistor 41. The voltage at terminal 42 will then be at I.sub.S.R41. Since the potential at terminal 42 is below ground, op-amp 40 must be provided with positive and negative supplies as shown.